Glazing units are known, such as automotive and architectural glazing units and the like, which provide selectively variable transparency to visible light. Glazing units comprising electro-active elements, such as a layer of electrochromic material or liquid crystal, have been suggested for this purpose. Variable transparency elements can be used, in the non-transparent mode, to provide privacy or to reduce glare and/or heating from sunlight or other light source. The substantially transparent condition or mode may be selected for vision through the glazing unit and to allow thermal energy to pass through to heat a space enclosed or partially enclosed by the glazing unit. Liquid crystal electro-active elements including polymer layers carrying the electrodes and encapsulating the liquid crystal material are commercially available as a premanufactured unit from Taliq Corp., Sunnyvale, Calif. It incorporates a nematic liquid crystal, having a curvilinear aligned phase, with ITO electrodes, encapsulated in a PET envelope. For automotive applications, a voltage converter is used to provide 100 volt A.C. current to the unit from an automotive 12 volt D.C. source. There are also liquid crystal materials operable with a 12 volt D.C. source without voltage conversion.
Means for preferentially reflecting non-visible radiation, particularly light in the infrared range of the spectrum, may be incorporated into a glazing unit. This is particularly advantageous if the structure is utilized as a motor vehicle sunroof or as an architectural window. Reducing the solar energy passing through a glazing unit is becoming increasingly important in architectural and automotive glazing units for reasons including improved environmental control and energy conservation. Particularly in the case of automotive applications, increased glazed surface area in the upper part of the vehicle body, that is, in the window and roof area, has resulted in greatly increased solar heating of the passenger compartment of the vehicle. This is especially so in the case of a so called "all glass" roof wherein substantially the entire roof and window (including windshield) area of the motor vehicle is transparent. Such all-glass roof would likely comprise glass and/or plastic exterior plys in a laminated structure. Such heating by solar energy generally is referred to as solar load and there is a growing demand for glazing units which significantly reduce solar load. Such solar load reduction ("SLR") feature, however, cannot be permitted to unduly diminish transparency to light in the visible wavelength range. This is particularly significant in the case of motor vehicle windows, especially windshields, which may be required by government regulations to have certain minimum transmission of visible light.
It is undesirable to handle increased solar load by increasing the size of the motor vehicle air conditioning unit, in view of the overriding need to reduce the size, weight and power consumption of motor vehicle components for increased fuel efficiency. In addition, the impending mandatory use of refrigerants less efficient than those currently in use increases the need to reduce solar load. Accordingly, the need to reduce the solar energy load passing through vehicle glazing has become especially critical.
Architectural and automotive glazing applications are now calling for glazing units having selectively variable transparency to visible light together with SLR functionality. Various teachings are known to those skilled in the art regarding glazing units providing variable transparency, solar load reduction or, at some level of efficacy, both these features. Prior to the present invention, however, known glazing units having acceptable manufacturing feasibility and costs have not provided adequate SLR functionality together with effective visible light transparency control. In U.S. Pat. No. 4,338,000 issued on Jul. 6, 1982 to Kamimori et al, a panel is described which is said to be both infrared reflecting and variably transparent. The Kamimori et al device includes a glass or plastic substrate carrying on its surface a tungsten oxide film covered by a metal film which is covered, in turn, by a second tungsten oxide film. A laminating layer of polyvinylbutyral ("PVB") laminates the film covered surface of the substrate to a second substrate which has an indium-tin oxide ("ITO") film on its laminated surface. The ITO film and the metal film are connected to an electrical power source to act as electrodes for the second tungsten oxide film, which is said to provide an electrochromic effect when an electrical potential is applied across the two electrodes. The metal film electrode is said also to reflect infrared radiation. It has been found, however, that this arrangement of a single metal film sandwiched between films of tungsten oxide provides inadequate solar load reduction to meet the stringent requirements of certain automotive and architectural glazing applications. In addition, the durability of the adhesion between the PVB laminating layer and an indium tin oxide film cannot be relied upon to adequately withstand prolonged exposure to sunlight, particularly the ultra-violet component thereof.
Various glazing units are shown in U.S. Pat. No. 4,749,261 issued on Jun. 7, 1988 to McLaughlin et al. The device of McLaughlin et al employs a liquid crystal sandwiched between electrodes and further comprises outer glass plys. The disclosure of the McLaughlin et al patent is incorporated herein by reference for its teachings regarding various elements suitable for use in the laminated vision panel of the present invention. A motor vehicle sunroof is shown to have an outermost glass ply laminated to a layer of PVB to act as a UV absorber, followed by an infrared reflector, followed by a first electrode, a liquid crystal and a second electrode, followed in turn by one or more layers of polyethylene terephthalate or glass. Optionally, the sunroof also comprises a de-polarizer layer and a polarizer layer. The efficacy of the infrared reflector layer in the McLaughlin et al device cannot be determined, since no particular materials for that layer are disclosed. In fact, the McLaughlin et al patent suggests that it may be desirable that the infrared reflective material also uniformly reduce the transmission of visible light. It is suggested that, optionally, the infrared reflector layer may be the first electrode for the liquid crystal. Suitable materials for that element are said to include a stainless steel coating or a tin oxide coating on the liquid crystal material. A layer of tin oxide, however, cannot be made thick enough to provide adequate SLR functionality. Even 50,000 Angstroms on clear glass would provide only slightly better IR rejection than the glass itself. A film of stainless steel, on the other hand, if it were of adequate thickness to provide adequate SLR functionality, would exclude far too much visible light for certain automotive and architectural applications. Also, manufacturing feasibility problems would be presented in providing a stainless steel or tin oxide coating directly onto a liquid crystal material in the McLaughlin et al device. In addition, the long term durability of the adhesion of the polyvinylbutyral to such electrode material is problematical. An alternative suggested in McLaughlin et al is to use a so-called low-E coating on the outer surface of the glass ply to reflect infrared energy. Known low-E coatings, however, would present durability issues if used on automotive glazing units in view of the abrasion and other difficult aspects of the use environment experienced by automotive glazing. In another embodiment said to be suitable for architectural purposes, an infrared reflective layer is shown on an outside surface of a layer of glass. The materials suggested for such layer, however, including ITO and tin oxide, do not provide sufficient preferential exclusion of infrared radiation verses visible light to meet the needs of various automotive and architectural applications. In addition, while McLaughlin et al suggest that the electrode for the liquid crystal material may serve also as the IR reflective material, the materials suggested for such combined part are, as discussed above, unable to meet the needs of various automotive and architectural applications.
It is an object of the present invention to provide a glazing unit having both means for selectively controlling transparency and, in addition, means for solar load reduction even when the vision panel is in a transparent mode. It is an object of certain preferred embodiments of the invention to provide a glazing unit which can be manufactured in accordance with commercially practical methods and technologies and which provides effective, full-time solar load reduction together with electro-active means for selectively controlling transparency to visible light. It is a particular object of certain most preferred embodiments of the invention to provide such a glazing unit which is suitable for use as a window in a motor vehicle.